1. Field of the Invention
The present invention relates to a low loss and stable optical wave-guide structure using a polymer material as the core of the optical wave-guides and their fabrication methods thereof.
2. Description of the Related Art
In order to attain high performance, high integration and low cost of an optical device, the research and development on the optical device having optical wave-guide structure has been actively carried out.
As the materials for the optical wave-guide, the following materials have been studied to be used: semiconductors (Si, GaAs, InP etc.), dielectric materials (ferroelectric materials such as LiNbO.sub.3, LiTaO.sub.3 ; glasses such as SiO.sub.2 glass group, compound glass group etc.; polymers such as polymethacrylic acid methyl, polystyrene, polycarbonate, polyimide etc.). For example, SiO.sub.2 is used as a material for optical wave-guide in Japanese Patent Application Laid-Open No.3-158802 (1991), polymer such as polyimide is used as a material for optical wave-guide in Japanese Patent Application Laid-Open No.3-188402 (1991) or Japanese Patent Application Laid-Open No.4-9807 (1992).
Among the above mentioned optical wave-guides, an optical wave-guide made of polymer has recently attracted great interest, since it can be fabricated with a simple process and in low cost. Therefore, the inventors of the present invention have studied a method shown in FIG. 8 in order to develop a method of fabricating polymer optical wave-guides.
Firstly, a buffer layer 2 made of a polymer material having a low refractive index (n.sub.b) is formed on a Si substrate 1 as shown in FIG. 8 (a).
Next, a core layer 3 (refractive index n.sub.w, n.sub.w &gt;n.sub.b) made of a polymer material is formed on the polymer buffer layer 2 as shown in FIG. 8 (b).
The buffer layer and the core layer described above are formed through spreading the polymers dissolved in a solvent by a spin coating method.
Then, a photoresist film is spread over the polymer core layer 3 as shown in FIG. 8 (c). After heating the photoresist film, a mask pattern is exposed on the photoresist by irradiating ultraviolet lays through a mask.
Next, a photoresist pattern 4 is formed on the polymer core layer 3 through developing, cleaning and drying.
Then, the polymer core layer 3 is formed in an approximately rectangle-shape as shown in FIG. 8 (d) through a dry etching process.
The dry etching process is carried out by using O.sub.2 gas or O.sub.2 gas mixed with a small amount of CF.sub.4 as an etching gas.
Finally, the optical wave-guide shown in FIG. 8 (e) is fabricated by forming a polymer cladding layer 5 having a refractive index of n.sub.c (n.sub.c =n.sub.b) throughout the surface of the polymer core layer 3.
However, it has been revealed that there are the following problems in fabricating a polymer optical wave-guide through the method shown in FIG. 8.
(1) In order to forming the optical wave-guide shown in FIG. 8 (e), the thickness of the polymer buffer layer 2 needs to be more than 8 .mu.m. Further, the thickness of the polymer core 3 needs to be several .mu.m to ten several .mu.m for a single-mode wave-guide, 10 .mu.m to dozens .mu.m for a multi-mode wave-guide. However, in the process in FIG. 8 (b), after forming the polymer buffer layer 2 made of polyimide having a thickness of more than 8 .mu.m, the polymer solution (polymer dissolved in a solvent) for forming a polyimide polymer core layer 3 having a thickness of more than 4 .mu.m is spread and heated at temperature of 90.degree. C. for 30 minutes, and the remaining solvent is removed. For the next step, when the polymer solution is heated at temperature of 200.degree. C. for 30 minutes, cracking occurs in the polymer core layer 3. A series of the experiments have been curried out with varying the heating time and the heating temperature. Cracking occurs in the every cases although there are differences in the amount of cracking produced. Studying the cause has revealed that [1] since the materials of the polymer buffer layer 2 and the polymer core layer 3 are the same and consequently the adhesion between them is strong, the strength of constriction of the polymer core layer 3 in the radial direction exceeds the strength of constriction in the thickness direction to cause the cracking; [2] since the thickness of the polymer core layer is too thick, the remaining stress becomes large to cause the cracking; [3] since the solvent in the polymer core 3 penetrates into the polymer buffer layer 2, cracking is caused on the surface of or in the inside of the polymer buffer layer 2, and further in the inside of the polymer core 3. PA1 (2) Optical wave-guides have been fabricated through the processes (c), (d) and (e) in FIG. 8 using the polymer core having cracking described above. The result is that the optical wave-guides have a large scattering loss due to non-uniformity in the structure.